I remembered someone mentioning
James Fenimore Cooper and I even found a book by him here.
https://www.mobileread.com/forums/showthread.php?t=10613
Another Western writer I remember from my childhood is
Mayne Reid, but I don't see any of his books uploaded.
Here's a site which has a few in HTML, html2lrf should work on them, I guess.
http://www.athelstane.co.uk/maynreid/index.htm
The Headless Horseman, which is probably his most famous book, doesn't seem to available online.